The invention relates to an aluminum alloy member that is used for joining the ends of a sheet-like aluminum alloy member by friction stir welding, and forming an anodic oxidation coating on the weld front surface or the weld back surface. More specifically, the invention relates to an aluminum alloy member that is used to produce a vehicular wheel or a housing.
A vehicular wheel and a housing are generally formed using an aluminum alloy member due to its light weight. For example, a vehicular wheel is produced by bending a sheet-like aluminum alloy member in the shape of a cylinder, integrally joining the ends of the sheet-like aluminum alloy member by friction stir welding, and forming each end of the resulting product by flaring. The surface opposite to the weld surface is used as a design surface, and an anodic oxidation coating is formed on the design surface in order to improve the corrosion resistance and the abrasion resistance of the design surface.
A housing is produced by integrally joining a side material and a covering material formed of an aluminum alloy member by friction stir welding, and smoothly facing the weld surface (including the weld) of the integrally joined side material and covering material. The weld surface is provided with an anodic oxidation coating in order to improve the corrosion resistance and the abrasion resistance of the weld surface, and is used as a design surface.
When producing a vehicular wheel or a housing as described above, the anodic oxidation coating formed on the design surface may show a difference in color tone between an area corresponding to the weld obtained by friction stir welding (i.e., an area including the weld and its peripheral area) and the remaining area (matrix (base material)) due to the difference in crystal grain size. In order to solve this problem, a technique that equalizes the crystal grain size by heat treatment has been proposed. However, even when the crystal grain size is equalized by heat treatment, the difference in color tone of the anodic oxidation coating may not be sufficiently reduced.
JP-A-2003-225780 and JP-A-2003-230970 disclose related-art technology.